In recent years, reduction in size and reduction in weight of mobile information terminals, such as a mobile phone, a notebook personal computer, and a smart phone, have been rapidly advanced, and a secondary battery used as a drive power source thereof has been further required to have a higher capacity. Since a non-aqueous electrolyte secondary battery which performs charge and discharge by transfer of lithium ions between a positive electrode and a negative electrode in association with charge and discharge has a high energy density and a high capacity, the secondary battery described above has been widely used as a drive power source of the mobile information terminal as described above.
Furthermore, recently, the non-aqueous electrolyte secondary battery has also drawn attention as a motive power source for an electric tool, an electric vehicle (EV), a hybrid electric vehicle (HEV or PHEV), or the like, and has been expected to be widely used in various fields.
Although the non-aqueous electrolyte secondary battery is used in a predetermined operational voltage range, if the operation is performed in an overcharged state beyond the voltage range, reactivity between a positive electrode and an electrolyte solution is increased. Accordingly, since decomposition of the positive electrode and an oxidation decomposition reaction of the electrolyte solution (non-aqueous electrolyte) occur, gas is generated, and the inside pressure of the battery is increased. When this state continues, for example, by Joule heat generation caused by an increase in internal resistance and a reaction heat caused by a chemical reaction between a decomposition product of the electrolyte solution and the positive electrode, the temperature of the battery is rapidly increased. As a result, problems, such as degradation in characteristics and safety of the battery, may arise.
In order to solve the problems as described above, for example, PTL 1 has suggested that by addition of a phosphoric acid compound to a non-aqueous electrolyte secondary battery positive electrode, an increase in battery temperature caused by a chemical reaction of an electrolyte solution in an overcharged state is suppressed.
In addition, PTL 2 has suggested that by adhering a phosphoric acid compound to the surface of a positive electrode active material, decomposition of an electrolyte solution in a high-temperature overcharged state is suppressed, and gas generation is suppressed.